1. Field of the Invention
The present invention relates to electrostatic and magnetic developing toners and more to a dual purpose toner useful in pressure fixing and/or thermal fixing development of latent images.
2. Description of the Prior Art
The function of toners in electrostatic reproduction is to be attractable by and adhere to an electrostatic image to form an initial visible image. More permanent adherence of the toner powder is attained by means of various fixing processes including heat and pressure. Early electrostatic developer mixes comprised a fine particle thermoplastic printing powder physically blended with a coarser charging medium called a "carrier". The carrier substance served to charge the printing particles by contact and provided a means to transport the toner to and from the development site. Recently, single-component toners have been introduced which are uncharged electrostatically in their normal state. The single component toner particles are electronically conductive and magnetically attractable. Electrostatic toner manufacturers are presently marketing two distinctly different single component toners; one for fixing by heat, or by a combination of heat and pressure, and a second type designed for fixing by the action of pressure alone.
The 3M VQC-1 Copier, for example, differs from other models in that it employs a combination of pressure and heat energy to fuse or fix toned images. On the other hand, 3M's VQC-II and VQC-III copiers use no heat for fixing. The latter machines depend solely upon a pair of highly polished steel rollers to physically calender or press the thermoplastic toner powder into the surface of the copy sheet. Substantial pressure is required to properly fix the image values of 150 to 200 pounds/inch have been reported.
U.S. Pat. No. 3,639,245 to Nelson and U.S. Pat. No. 3,925,219 to Strong describe a method of manufacturing toner in which the toner is compounded, finely ground and then spheroidized to produce particles that are essentially spherical in shape for use in the 3M VQC-I pressure and heat fixing machines. The actual products marketed by the 3M company (catalog no. 361 and 365) consist of particles that are truly spherical. Nelson at column 5, line 20 and Strong at column 4, line 50 disclose the use of a 700.degree.-800.degree. F. air stream which is at a temperature capable of at least softening and desirably melting the plastic resin.
Truly spherical particles can also be manufactured by spray drying, either by solvent spray drying or spray chilling. In solvent spray drying the toner resins are completely dissolved in a suitable solvent and then mixed with other dyes, pigments and additives. The dispersion is then pumped or fed by gravity to the top of a spray dryer. Upon entering the spray dryer the liquid mixture is atomized by dropwise feed onto a rapidly spinning disc, by passing through a specially constructed spray nozzle, or by other conventional means. The atomized cloud of fine liquid particles falls vertically downward into the main chamber of the spray dryer. A heated stream of air is fed at a 90.degree. angle to the spray. Because it enters the cylindrical chamber tangentially, the air spirals down through the equipment in cyclone fashion. The swirling motion causes the particles to assume a spherical shape. In this respect, the spray dryer resembles a shot tower. Spray drying is expensive and unless fitted with a solvent recovery or incineration unit, is environmentally objectionable because of the large amounts of solvent vapors released to the environment.